1. Field of the Invention
The present invention relates to catalyst powder suitable for an exhaust gas purifying catalyst which purifies exhaust gas discharged from an internal combustion engine, and to a method of producing the catalyst powder.
2. Description of the Related Art
In recent years, research and development on a catalyst which purifies exhaust gas have been progressed in various ways. As such an exhaust gas purifying catalyst, there is one which carries, as catalyst activators, noble metal such as platinum (Pt) and palladium (Pd) and transition metal such as cobalt (Co) and cerium (Ce) on a carrier formed of a porous body of alumina (Al2O3) or the like. As a method of carrying the catalyst activators on the carrier, there is an impregnation method, an SPC method and the like.
In the impregnation method, the catalyst activators such as the noble metal and the transition metal are first dissolved in pure water to make an aqueous solution, the carrier is immersed in the aqueous solution to adsorb the noble metal and the like onto the carrier, followed by drying and baking, thereby preparing catalyst powder. However, an adsorption site on a surface of the carrier is limited, and accordingly, there have been limitations on an improvement of dispersivity of the catalyst activators on the carrier surface. Therefore, the catalyst activators cannot be highly dispersed, and it has been thus impossible to obtain a high catalytic activity.
In this connection, a method of enhancing the catalytic activity has been developed, in which the catalyst activators such as the noble metal carried on the carrier is atomized to increase specific surface areas thereof. For example, a method of producing the catalyst powder is disclosed, in which the catalyst activators are prepared in micelles by use of a reversed micelle method (microemulsion) (Japanese Patent Laid-Open Publication No. 2000-42411). In this technology, first, a micelle containing the noble metal therein and a micelle containing the other element such as the transition metal therein are individually prepared. Subsequently, the respective micelles individually prepared are dropped in a solution containing aluminum isopropoxide as a carrier precursor, and an obtained solution is subjected to hydrolysis, the baking and the like, thereby preparing the catalyst powder.